Pourous fluorocarbon membrane, method for producing thereof and cartridge filter based on said membrane

ABSTRACT

A porous fluorocarbon membrane has a porous support completely impregnated with tetrafluoroethylene/vinylidene fluoride copolymer. There is a process for producing this membrane by impregnating a porous support with a copolymer solution in a readily-boiling solvent and a precipitating mixture of alcohol and water followed by curing for partial hardening of the solution and subsequent drying at elevated temperature. A cartridge filter includes protective and supporting cylindrical perforated hollow housings, containing a fluorocarbon membrane or two superimposed flourocarbon membranes. Membranes and a cartridge filter find extensive use in separation of gases and aggressive media.

FIELD OF THE INVENTION

The present invention relates to a porous fluorocarbon membrane, methodfor production thereof and design of cartridge filter based on a porousfluorocarbon membrane. These filters are widely used in fine filtrationof chemically active media due to their resistance with respect toagressive media and organic solvents commonly in use. Fluorocarbonmembranes and cartridge filters based on said membranes can beconveniently sterylized by thermal treatment since they offer therequired thermal stability. At the same time, porous fluorocarbonmembranes are inherently hydrophobic, which is a disadvantage in theprocess of filtration of water-containing aqueous media. However,numerous techniques, well-known in the art, can be used to renderfluorocarbon membranes hydrophilic. Therefore, said disadvantage can bereadily overcome.

PRIOR ART

Porous fluorocarbon membranes produced from the solution oftetrafluoroethylene/vinylidene fluoride copolymer using both "wet" and"dry" processes, are well-known to those skilled in the art. "Wet"process is most common. First step thereof includes preparation of acasting (working) solution, consisting of a fluorocarbon polymer,solvent and non-solvent for said polymer. The working solution is thencast on a dense surface, which is unable to absorb said solution.Normally glass, polyethylene terephthalate film, stainless steel drum,etc. are used as said surface. At this point, either one of two ways canbe employed: cast solution can be allowed to evaporate (in air or othercontrolled environment), within a specified period of time varying froma few seconds to a few minutes, after which the solution (already partlyhardened) may be immersed in a quench bath. Alternatively, membranecasting can be performed directly under the surface of the quench bath,thus completely eliminating the stage of partial evaporation andhardening of the solution. Both methods are used on commercial scaleproviding membranes principally characterized by low strength andperformance (service life and the amount of impurities retained). Theabove disadvantages encountered in "wet" process for producing porousmembrane can be eliminated by using porous support in "wet" process andin membrane design, respectively. In this case porous support acts as asurface coated by a casting solution of fluorocarbon polymer. At thispoint a new problem emerges, i.e. rate of "wet" casting of membrane onporous support is drastically reduced and reaches only 0.52-1.0 m/min.

In another well-known "dry" process for producing porous fluorocarbonmembrane, based on casting solution of tetrafluoroethylene(TFE)/vinylidene fluoride (VF) copolymer, said solution is formed intomembrane on dense surface followed by complete evaporation of liquidcomponents of the casting solution and peeling the membrane sheet fromsaid surface. Disadvantages of said process are similar to thoseencountered in "wet" process, wherein porous support is not used, i.e.low mechanical strength of membrane and relatively low performancethereof. "Dry" process for producing porous fluorocarbon membranes onporous support has not yet been disclosed in prior art.

Another common method for producing porous membranes is known as"thermal" method. This method has been developed for the production ofporous membranes from fluorocarbon polymers which are hard to dissolvein the available solvents and at temperatures acceptable forcommercial-scale production.

Representative of this genre is U.S. Pat. No. 4,990,294 issued to Yene.a., which discloses the preparation of blend from fluorocarbon polymerand solvent melt followed by coating the casting surface with theresultant blend from the extruder. The solvent is then removed bylowering the temperature. Disadvantages of said method are as follows:high-temperature destruction of polymer in the course of preparing meltthereof, relatively low rate of porous membrane formation and lowstrength of said membrane.

An example of "wet" process for producing porous fluorocarbon membranesis-disclosed in U.S. Pat. No. 4,384,047. This patent discloses porousfluorocarbon membrane based on TFE/VF copolymer, which membrane isproduced from casting solution on dense surface followed by partialevaporation and hardening of casting solution and placement of thusobtained membrane in liquid gelation (precipitating) medium.

Another example of "wet" process for producing porous fluorocarbonmembrane based on TFE/VF copolymer is U.S. Pat. No. 4,965,291. Inaccordance with this patent copolymer casting solution is formed intomembrane directly under the surface of liquid setting bath, thuseliminating the stage of partial evaporation and hardening of thesolution.

An example of "dry" method for producing unsupported porous fluorocarbonmembranes is German Patent application (DE. A. 2,632,185) based on aJapanese Patent application filed Jul. 17, 1975, issued to Mano,Hitoshi, e.a. This patent discloses a method for producing porousfluorocarbon membranes by dissolving TFE/VF copolymer; preparing castingsolution, applying said solution onto dense plate, evaporating liquidcomponents of said solution to form porous membrane and peeling saidmembrane from the plate.

Porous fluorocarbon membranes are widely used in filtration unitsdesigned in the form of a cartridge filter. As a rule, cartridge filterincludes protective and supporting cylindrical perforated hollowhousings, pleated porous membrane provided with prefiltration device anddrainage placed between said housings and tightly connected to the casesof end caps, at least one of said caps is provided with a central drainduct. In most cases cartridge filters differ in design of porousmembrane used therein.

U.S. Pat. No. 4,929,354 discloses the design of cartridge filterprovided with membrane made of aliphatic polyamide. Said cartridgefilter exhibits low performance.

DETAILED DESCRIPTION OF THE INVENTION

The object of the present invention is to provide porous fluorocarbonmembrane and cartridge filter based on said membrane which enable toperform the process of filtration of chemically active media exhibitingthe best process and economical characteristics. In particular, theobject of the present invention is to provide commercially applicableeconomical high-efficient method for producing said fluorocarbonmembrane.

The porous fluorocarbon membrane according to the present invention iscomprised of:

porous support having the average pore size of 5.0 to 500.0-microns anddifference by specific weight in any two points of a sample havingvarious sizes of 7.0% max., which support is completely impregnated by acopolymer, soluble in organic solvent, wherein said copolymer comprises23-25 wt. % of tetrafluoroethylene and 75-77 wt. % of vinylidenefluoride and is characterized by (which characteristic defines themolecular weight and strength of copolymer being used): ratio ofviscosity of copolymer solution in acetone to acetone equal to 2.0-4.0with the copolymer content in the solution in the amount of 0.01 g/cm³.

It was disclosed that the performance of the porous fluorocarbonmembrane could be improved to a considerable extent provided:

a) a membrane design includes porous support having presetcharacteristics, determining structure thereof: average porosity anddifference by specific weight in one sample

b) a membrane design includes support completely impregnated (coated) byTFE/VF copolymer in "dry" process for membrane production

c) a membrane design includes porous support completely impregnated byfluorocarbon copolymer having specified composition, molecular weightand strength: TFE/VF ratio, viscosity ratios.

The design of porous fluorocarbon membrane of the present invention ischaracterized by improved strength and performance. Due to saidcharacteristics, intrinsic to the initial membrane, design of cartridgefilter according to the present invention also displays improvedperformance. In accordance with the present invention, along withemploying TFE/VF copolymer, it is possible to use the blend of saidcopolymer with other fluorocarbon polymers in the amount of 15% wt.max., i.e. at least 85% by weight of said copolymer is used. Thefollowing fluorocarbon polymers may be used as an additive to the basicTFE/VF copolymer: polyvinylidene fluoride, vinylidenefluoride/trichloroethylene copolymer and hexafluoropropylene/vinylidenefluoride copolymer. Said additives contribute to the improvement inporous membrane performance.

Porous fluorocarbon membrane of the present invention is produced by"dry" method, which in accordance with the invention includes:dissolution of TFE/VF copolymer in organic readily-boiling solvent;mixing of the resultant copolymer solution with precipitating mixture toform casting solution, formation of the resulting casting solution intomembrane on porous material, i.e. the support, having the average poresize of 5.0 to 500.0 microns and 7% max. difference by specific weightin any two points of a sample having various sizes. Said membraneformation is performed by complete impregnation of porous material, i.e.support, with heated casting solution of copolymer, consisting of 23-25%by weight tetrafluoroethylene and 75-77% by weight vinylidene fluorideand having the ratio of viscosity of solution in acetone to acetoneequal to 2-4, with copolymer content in the solution in the amount of0.01 g/cm³, by short-term storage of membrane under formation in theconditions of free evaporation of the part of solvent for partialhardening of casting solution and subsequent drying of the resultingmembrane in at least three successive drying zones, wherein temperatureis maintened such that it increases from one zone to another.

In accordance with "dry" process of the present invention for producingporous fluorocarbon membrane based on porous support, ketones, e.g.methyl ketone and acetone, are preferrably used as readily-boilingorganic solvent for fluorocarbon polymer (mixture of fluorocarbonpolymers). When using acetone, which is a preferred embodiment, it istaken in the amount of 60-70% by weight of the weight of castingsolution, wherein concentration of fluorocarbon polymer itself is from 7to 12% by weight.

According to the present invention alcohol/water mixture is used as aprecipitating mixture. Ethanol, buthanol, propanol and isopropanol mayalso be used. However, isopropanol or propanol are preferably used inthe amount of at least 14% by weight, practically from 14 to 17% byweight, based on casting solution weight. Deionized water is used aswater in the amount of at least 5.0% by weight, practically from 5 to10% by weight, based on casting solution weight. Water is preferablydeionized to the point where metal and salt content is less than 2.0g/m³.

In accordance with the method of the present invention dissolution offluorocarbon polymer, in particular, said tetrafluoroethylene/vinylidenefluoride copolymer, is performed at elevated temperature, preferably at30° C.-50° C.

In accordance with the method of the present invention precipitatingmixture is produced separately from fluorocarbon polymer solution bymixing deionized water and alcohol at the temperature ranging from 18°C. to 25° C., under the pressure of inert gas, preferably nitrogen,ranging from 2 to 2 kPa.

According to the method of the present invention further mixing offluorocarbon polymer solution and precipitating mixture is performed atelevated temperature, preferably at 45° C.±5° C. It is important to notethat when precipitating mixture is fed into reactor containing heatedfluorocarbon polymer solution, in accordance with the invention, thetemperature difference between fluorocarbon polymer solution andprecipitating mixture should not exceed 3° C. Failure to observe saidcondition will make processing of the resulting casting solution intohigh-quality membrane more complicated.

Preferred embodiment of the complete impregnation of porous support withcasting solution includes the employment of application and sizingrollers having different rotational speed. Said rollers are partlysubmerged into casting solution. During rotation said rollers pick upcasting solution and convey it onto porous support moving at a constantspeed ranging from 1.0 to 10.0 m/min. Porous support is in constantcontact with application roller. For the purpose of this inventioncontinuous band made of porous material, meeting above mentionedspecifications, is used as porous support. Support is preferably porousweb of non-woven polypropylene 0.011-0.016 mm thick, 12-14 inches wide(300-350 mm), having surface density of 50-70 g/m². In practice, anyporous material meeting the above requirements may also be used. Porouspolyethylene terephthalate (lavsan) web having surface density of 35-70g/m² has proven its suitability.

Composition of casting solution used for producing fluorocarbon porousmembrane in accordance with the method of the present invention isselected proceeding from the required porosity of membrane and equipmentcurrently in use. In case of roller method for application andpreparation of casting solution the composition of casting solutionpresented in Table 1 is preferred.

                  TABLE 1                                                         ______________________________________                                        Suggested casting solution composition                                        (% wt. based on 100% content)                                                 Pore size in                                                                  membrane,                                                                              Copolymer  Acetone   Water  Isopropanol                              microns  content    content   content                                                                              content                                  ______________________________________                                        0.1      10.5-11.5  67.7-65.3 6.5-8.2                                                                              15.3-15.0                                0.2       9.5-10.5  68.8-64.9 6.5-8.6                                                                              15.2-16.0                                 0.45    8.5-9.5    68.1-67.8 7.0-8.0                                                                              16.4-14.7                                 0.65    7.5-8.5    69.1-68.1 7.0-8.2                                                                              16.4-15.2                                ______________________________________                                    

In accordance with the present invention a casting solution is appliedon a porous support based on 10-50 g of copolymer per 1 m² of poroussupport. The temperature of casting solution is maintained within therange of 25° C.-45° C.

After the application of casting solution onto porous support iscomplete, said support becomes completely impregnated with said solutionand is subjected to a short-term storage in the conditions of freeevaporation of a portion of solvent at 18° C.-25° C. within 0.5-1.0 min.Under said conditions partial hardening of the casting solution occursand a porous membrane starts to form.

Membrane formation process is completed at the stage of drying, which inaccordance with the present invention, is carried out at least in threesuccessive drying zones. Various temperatures are maintained in saidzones, which temperatures increase as the resulting membrane istransferred from zone to zone, absolute humidity in said zones ismaintained at approximately similar level. Preferred temperature valuesin drying zones when using roller continuous method for producingmembrane on moving web of porous support are as follows:

first zone--ranging from 45° C. to 55° C.

second zone--from 55° C. to 65° C.

third zone--from 90° C. to 100° C.

Drying results in porous fluorocarbon membrane exhibiting high strengthand performance which makes it possible to use said membrane in acartridge fitter therefore endowing said filter with a novelfeature--exclusive reliability and long service life while maintaininghigh filterability.

Design of the cartridge filter in accordance with the present invention(FIG. 4) consists of protective (33) and supporting (34) perforatedhollow housings, pleated porous membrane (35) equipped withprefiltration device (36) and drainage (37) placed between said housingsand tightly connected to the cases of end caps (38), at least one ofsaid caps is provided with a central drain duct. The cartridge filter ofthe present invention includes a membrane consisting of a porous supporthaving average pore size in the range of 5.0-500.0 microns and 7% max.difference by specific weight in any two points of a sample havingvarious sizes and which is completely impregnated with copolymer solublein organic solvent, which copolymer contains 23-25% by weighttetrafluoroethylene and 75-77% by weight vinylidene fluoride and whichcopolymer has the ratio of viscosity of solution in acetone to acetoneequal to 2.0-4.0, calculated with copolymer content in the solution inthe amount of 0.01 g/cm³.

In accordance with the present invention said impregnation may becarried out by the mixture of said copolymer with other modifyingfluorocarbon additives. Content of said additives must not exceed 15weight percent of total amount of fluorocarbon polymer. Polyvinylidenefluoride, vinylidene fluoride/trifluorochlorethylene copolymer orhexafluoropropylene/vinylidene fluoride copolymer may be used as amodifying additive.

In order to improve performance of the cartridge filter, i.e. degree ofcleaning, amount of impurities retained, reliability and durability, thedesign of said filter, according to the invention, includes twospecified porous fluorocarbon membranes superimposed such that membranewhich is closer to prefiltration device has pore size ranging from 0.25to 0.65 microns and the second membrane has pore size in the range of0.15-0.45 microns. Said superimposition of membranes, in accordance withthe invention, may be performed by two ways: either by active membranesurfaces in the direction of prefiltration device, or by surfacesopposite to active membrane surfaces. In the latter case reliability ofcartridge filter is improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1--illustrates the stages of the experimental continuous method forproducing fluorocarbon porous membrane in accordance with the process ofthe invention

FIG. 2--illustrates an anchor mixer used for casting solutionpreparation in the Examples of experimental production of membrane ofthe present invention

FIG. 3--illustrates lay-out of basic equipment used for the applicationof casting solution onto porous support in the Examples using theexperimental continuous method for producing membrane of the presentinvention

FIG. 4--general view of the cartridge filter containing membrane of thepresent invention

FIGS. 1-4: Symbols:

1--measuring tank for the solvent for fluorocarbon polymer (copolymerand mixtures thereof)

2--measuring tank for alcohol included into precipitating mixture

3--deionized water meter

4--reactor--mixer for preparation of precipitating mixture

5--reactor for preparation of the solution of fluorocarbon polymer(copolymer and mixtures thereof) and casting solution

6--gear pump

7, 8, 10, 11--valves

9--filter 12--deaerator

13--drying chamber

14--porous support

15--casting roll assembly

16--finished porous fluorocarbon membrane

17--take-up reel

18--stiffening rib

19--blade of mobile mixer

20--blade of internal fixed mixer

21--mobile mixer

22--drive shaft of fixed mixer

23--flange

24--internal fixed mixer

25--reactor cover

26--joint assembly of internal fixed mixer

27--application roller

28--tension rollers

29--casting solution bath

30--casting solution

31--sizing roller

32--doctor blade

33--protective perforated hollow housing

34--supporting perforated hollow housing

35--porous fluorocarbon pleated membrane

36--pleated prefiltration device

37--pleated drainage

38--end cap

Preferred Embodiment of the Invention

Design of a porous fluorocarbon membrane, a method for producing thereofand a cartridge filter based on said membrane have been tested on pilotscale. Embodiments presented herein were performed using a pilot plantoperating in continuous mode. Lay-out of said plant is shown in FIG. 1.

Liquid components involved in the process, i.e. solvent (ketone),alcohol and deionized water are fed to the appropriate reactors frommeasuring tanks 1,2,3. Alcohol (propyl or isopropyl) and deionized waterare fed to the reactor 4 for the preparation of precipitating mixture.The reactor 4 is equipped with a heating jacket. Preparation ofprecipitating mixture is effected by mixing alcohol and water at roomtemperature (within the range of 18° C.-25° C.), nitrogen pressure (from2.0 to 3.0 kPa). Rotational speed of the mixer is approximately 48±2rev/ min. Mixing time is about 30±5 min.

Copolymer solution (or mixture thereof with other fluorocarbon polymers)and casting solution are prepared in the reactor--mixer 5, having thecapacity of 40.0 I, provided with a heating jacket and an anchor mixer,as shown in FIG. 2. First, approximately two-thirds of the designedamount of a solvent are delivered to the reactor 5 from the measuringtank 1. The mixer is then started and the designed amount offluorocarbon is charged uniformly for approximately 25±5 min. On thecompletion of fluorocarbon polymer charging the remaining amount ofsolvent is fed from the measuring tank 1 to the reactor 5. After thatheating of the heat carrier in the reactor 5 jacket is turned on and thecontent thereof is heated up to 45±5° C. Stirring is continued for 1hour. Then heating of circuit pipes connecting the equipment4-5,5-9,9-12, 12-15 is started. Valve 7 is opened (while valves 8,10 and11 remain closed) and the gear pump 6 is put into operation. Stirring ofthe reactor 5 content is continued for about 3.0 hours while the mixerand the circuit are running. Thus, total time required for fluorocarbondissolution is about 4.0 hours.

Approximately 1.0 hour prior to the completion of fluorocarbon polymerdissolution heating of the heat carrier in the reactor--mixer 4 jacketis started and precipitating mixture, contained in the reactor 4, isheated up to 45±5° C. while the mixer is running.

On the completion of fluorocarbon polymer dissolution the precipitatingmixture from the reactor 4 is fed to the reactor 5 uniformly for about1.5+0.25 h. Stirring is then continued at 45±5° C. Total time requiredfor the delivery of precipitating mixture and dissolution offluorocarbon polymer ranges from 4 to 6.0 hours.

It should be noted that when feeding the precipitating mixture to thereactor 5, already containing the solution of fluorocarbon polymer,temperature difference between fluorocarbon polymer solution andprecipitating mixture must not exceed 3° C.; otherwise quality of theresulting membrane is deteriorated. Delivery of the precipitatingmixture to the reactor 5 is performed under continuous stirring andcirculation of the resulting casting solution by means of pump 6.

The resulting casting solution is a system thermodynamically unstable atroom temperature. Storage life of the finished casting solution at 45±5°C. under nitrogen pressure ranging from 2.0 to 3.0 kPa in a tightlysealed container is about 5 days.

Prior to the casting solution delivery to casting roll assembly (pos.15, FIG. 1), filtration and deaeration thereof are preferred. Filtrationof the casting solution is effected by means of the filter 9 and thecircuit involving valves 7,8,10 and 11. Preferable fineness offiltration is about 1.0 micron.

Deaeration of the casting solution is performed for at least 4.0 hoursat 40±5° C. by sedimentation in the deaerator 12.

Filtered and deaerated casting solution is then fed to the casting rollassembly 15, as shown in FIG. 3.

The casting solution of fluorocarbon polymer is applied onto thecontinuously moving porous support by means of the rotative applicationroller 27 and the rotative sizing roller 31 partly submerged into thecasting solution bath 29. In the course of the casting solutionapplication the support 14 is transported at constant speed, rangingfrom 1.0 to 10.0 m/min, over the application roller 27, which roller isin close contact with said support. It is necessary that an even layerof the casting solution in the amount of from 10 to 50 g of fluorocarbonpolymer per 1 m² of porous support is applied on said support.

The thickness of the applicable layer of the casting solution iscontrolled by the distance (clearance -δ--) fixed between theapplication roller 27 and the sizing roller 31. Doctor blade 32, whichblades are tightly pressed against the surface of the sizing roller 31,contributes to more precise application of the required amount ofcasting solution onto the support. Clearance -δ-- ranging from 700 to1200 microns is preferable. However, the precise value thereof isdetermined in each specific case with respect to the equipment in use.

The ratio of linear rotational speeds of the application roller 27 andthe sizing roller 31 as well as the speed of porous support 14 movementeffect the thickness of the applied layer of the casting solution.Linear rotational speed of the application roller 27 (diameter thereofis in the range of 100-120 mm) must be 2.0 to 10.0 m/min. Linearrotational speed of the sizing roller 31 (diameter thereof is in therange of 100-120 mm) must be 1.0 to 5.0 m/min. The support transportingspeed ranges from 1.0 to 10.0 m/min.

Temperature of the casting solution in the bath 29, in the course ofapplication onto porous support, is maintained at 25° C.-45° C.

Continuously moving support 14, coated with the casting solution, isthen transferred to the arch-type three-zone drying chamber (pos. 13,FIG. 1). While continuously transferring the support, coating thereof issubjected to short-term storage at the temperatures ranging from 18° C.to 25° C. for about 0.5-1.0 min. Within this time partial hardening ofthe casting solution occurs and a porous membrane starts to form. Theresulting continuously moving membrane is then transferred to the drier13, provided with three drying sections (zones) wherein temperatures andhumidity are maintained within the following ranges:

first zone--from 45° C. to 55° C.

second zone--from 55° C. to 65° C.

third zone--from 90° C. to 100° C.

Approximately same absolute humidity equal to 5.0 g/m³ max. ismaintained in all three zones.

In accordance with the present invention the most mild conditions forthe removal (evaporation) of liquid components of the casting solutionare maintained in the first zone, which contributes to the formation ofpolymer gel characterized by the required intensity. Properties of saidgel produce strong effect on the formation of the porous membranestructure. The second zone is the zone of the resultant gel syneresisand formation of the porous membrane structure. In the third zone (FIG.1), residual liquid components are finally removed from the castingsolution and a porous structure of membrane is fixed.

Table 2 summarizes approximate process parameters of air supplied to thezones of the drying chamber 13. However, it should be noted that optimumvalues are to be selected depending on the specific type of equipment inuse.

                  TABLE 2                                                         ______________________________________                                        Characteristics of air per zones in the drying chamber                        Zone,                          Absolute humidity,                             No.  Air flow, m.sup.3 /h                                                                      Air temperature, ° C.                                                                g/m.sup.3                                      ______________________________________                                         1   2000 ± 100                                                                             50 ± 5     5.0 max.                                        11  2000 ± 100                                                                             60 ± 5     5.0 max.                                       111  4000 ± 200                                                                             95 ± 5     5.0 max.                                       ______________________________________                                    

After leaving the drying chamber 13 the resultant porous membrane iswound on the reel 17.

Table 3 illustrates specific embodiments for producing a porousfluorocarbon membrane of the present invention, characteristics thereofand characteristics of a cartridge filter based on said membrane andproduced in accordance with the invention.

While there have been described various embodiments of the presentinvention, considered to be preferred embodiments at present, it will beobvious to those skilled in the art that various changes andmodifications may be made therein without departing from the inventionand it is, therefore, aimed to cover all such changes and modificationsas fall within the true spirit and scope of the invention.

Industrial Applicability

The present invention may be used in order to update current or createnovel commercial processes for producing porous fluorocarbon membranesand cartridge filters based on said membranes.

Updating can enable the implementation of the commercial, totally "dry"method for producing porous supported fluorocarbon membranes. Membranesof this type offer high and stable performance which contributes to theproduction of high-quality and reliable cartridge filters.

                                      TABLE 3                                     __________________________________________________________________________    Embodiments of the present invention on pilot scale                           __________________________________________________________________________                                                 Temperature                      copolymer composition,                       of copolymer                     % by weight          composition of casting solution, % by                                                                 dissolution                      No.                                                                              TFE   VF   viscosity ratio                                                                      copolymer                                                                          ketone  water                                                                            alcohol ° C.                      __________________________________________________________________________     1 25    75   2.9    10   68.25 acetone                                                                         6.52                                                                             15.23 i-propanol                                                                      45                                2 23    77   2.8    10                      45                                3 23    77   3.1    10   68.25 m-e-ketone                                    .- 15.23 propanol                                                                      45                                                                    4 23    77   2.8    11.5 60 acetone                                                                            8.55                                                                             19.95 i-propanol                                                                      45                                5 23    77   2.8    9.5  69 acetone                                                                            7.5                                                                              14.0 i-propanol                                                                       45                                6 23    77   2.8    10.5 69.5 acetone                                                                          5.0                                                                              15.0 i-propanol                                                                       45                                7 23    77   2.8    10.0 68.25 acetone                                                                         6.52                                                                             15.23 i-propanol                                                                      40                                8 23    77   2.8                            50                                9 23    77   2.8                            45                               10 23    77   2.8                            45                               11 23    77   2.8                            45                               12 23    77   2.8                            45                               13 23    77   2.8                            45                               14 23    77   2.8                            45                               15 23    77   2.8                            45                               16 23    77   2.8                            45                               17 23    77   2.8                            45                               18 23    77   2.8                            45                               19 23    77   2.8    11.5 63.5    8.2                                                                              15.0    45                               20 23    77   2.8    7.5  69.1    7.0                                                                              16.4    45                               __________________________________________________________________________     Notes: TFE  tetrafluoroethylene: VF  vinylidene fluoride; me-ketone           methyl ethyl ketone                                                      

                  conditions for application of casting solution                  conditions for casting                                                                      onto support**      conditions for 3-stage                      solution preparation, ° C.                                                           speed,                                                                             consumption                                                                          solution                                                                              drying ° C. per                                                                  humidity,                         No.                                                                              solution                                                                           precipitator                                                                        m/min                                                                              rate, g/m.sup.2                                                                      temperature, ° C.                                                              I  III III                                                                              g/cm.sup.3                        __________________________________________________________________________     1 45   45    3    30     40      50 60  90 4                                  2 45   45    3    30     40      50 60  90 4                                  3 45   45    3    30     40      50 60  90 4                                  4 45   45    3    30     40      50 60  90 4                                  5 45   45    3    30     40      50 60  90 4                                  6 45   45    3    30     40      50 60  90 4                                  7 40   40    3    30     40      50 60  90 4                                  8 50   50    3    30     40      50 60  90 4                                  9 45   42    3    30     40      50 60   90T                                                                             4                                 10 45   45    10   30     40      50 60  90 4                                 11 45   45    4    30     40      50 60  90 4                                 12 45   45    3    10     40      50 60  90 4                                 13 45   45    3    50     40      50 60  90 4                                 14 45   45    3    30     35      50 60  90 4                                 15 45   45    3    30     45      50 60  90 4                                 16 45   45    3    30     40      45 55  90 4                                 17 45   45    3    30     40      55 65  100                                                                              4                                 18 45   45    3    30     40      50 60  90 5                                 19 45   45    3    30     40      50 60  90 4                                 20 45   45    3    30     40      50 60  90 4                                 __________________________________________________________________________     Notes:                                                                        ** "speed" means "speed of support": "consumption rate" means "consumptio     of copolymer": "solution temperature" means "temperature of casting           solution at the point of application thereof onto moving support         

                                      characteristics of cartridge filter                                           having                                                                        effective, surface of 0.4 m.sup.2                            membrane characteristics                                                                       pore size                                                                          bubble point,                                                                       ethanol flux,                    No.                                                                              porous support material                                                                     bubble point, atm                                                                     ethanol flux, l/m.sup.2 h                                                              microns                                                                            atm   l/m.sup.2 h                      __________________________________________________________________________     1 porous. polypropylene (60/7)***                                                             1.5     3000     not defined                                  2 porous polypropylene (70/7)                                                                 1.4     4000                                                  3 lavsan        1.3     3000                                                  4 porous polypropylene (70/7)                                                                 1.7     1300                                                  5 porous polypropylene (70/7)                                                                 0.8     11000    0.45 0.65  3100                              6 porous polypropylene (70/7)                                                                 2.7     1800     not defined                                  7 porous polypropylene (70/7)                                                                 1.4     4000                                                  8 porous polypropylene (70/7)                                                                 1.4     4000                                                  9 porous polypropylene (70/7)                                                                 1.4     4000                                                 10 porous polypropylene (70/7)                                                                 0.8     10000    0.20 0.95  1050                             11 porous polypropylene (70/7)                                                                 1.3     4000                                                 12 porous polypropylene (70/7)                                                                 0.3     14000    not defined                                 13 porous polypropylene (70/7)                                                                 2.0     1700     0.15 1.55   510                             14 porous polypropylene (70/7)                                                                 1.4     4000     not defined                                 15 porous polypropylene (70/7)                                                                 1.4     4000                                                 16 porous polypropylene (70/7)                                                                 1.4     4000                                                 17 porous polypropylene (70/7)                                                                 1.4     4000                                                 18 porous polypropylene (70/7)                                                                 1.4     4000                                                 19 porous polypropylene (70/7)                                                                 2.5     1000     0.10 1.95   330                             20 porous polypropylene (70/7)                                                                 0.2     75000    not defined                                 __________________________________________________________________________     Notes:                                                                        ***Values in brackets specify pore size of porous material (in microns)       and difference by specific weight in any two points of a sample having        various sizes of porous material (in %)                                  

What is claimed is:
 1. A porous fluorocarbon membrane comprisinga poroussupport and a layer of fluorocarbon polymer connected to said poroussupport, said porous support having an average pore size ranging from5.0 to 500.0 microns and difference by specific weight in any two pointsof a sample having various sizes of not more than 7%; said poroussupport is completely impregnated with a copolymer solution soluble inorganic solvent wherein the copolymer is consisting of 23-25 weightpercent tetrafluoroethylene and 75-77 weight percent vinylidenefluoride, the ratio of viscosity of the copolymer solution in acetone,with copolymer content in the solution in the amount of 0.01 g/cm³, toacetone equal to 2.0-4.0.
 2. A porous fluorocarbon membrane of claim1,wherein said membrane contains, as fluorocarbon polymer, the mixtureof polymers comprised of at least 85.0 weight percenttetrafluoroethylene/vinylidene fluoride copolymer and fluorocarbonpolymer selected from the group consisting of polyvinylidene fluoride,vinylidene fluoride/trifluorochloroethylene copolymer andhexafluoropropylene/vinylidene fluoride copolymer.
 3. A method forproducing a porous fluorocarbon membrane by dissolvingtetrafluoroethylene/vinylidene fluoride copolymer in an organicreadily-boiling solvent, mixing the resultant copolymer solution with aprecipitating mixture to produce a casting solution, forming saidcasting solution into a membrane on a continuously moving carryingsurface and drying the resulting continuously moving membrane atelevated temperature, wherein porous material having the average poresize within the range of 5.0-500.0 microns and difference by specificweight in any two points of a sample having various sizes of not morethan 7% is used as a carrying surface; formation is performed by thecomplete impregnation of porous material moving at a constant speed withheated casting solution of a copolymer comprising 23-25tetrafluoroethylene and 75-77 weight percent vinylidene fluoride andhaving the ratio of viscosity of the copolymer solution in acetone, withcopolymer content in the solution in the amount of 0.01 g/cm³, toacetone equal to 2.0-4.0; short-term storage of continuously movingporous material while applying the casting solution under the conditionsof free evaporation of a portion of solvent for partial hardening of thecasting solution and consequent drying of continuously moving porousmaterial is effected in at least three successive drying zones whereintemperature, increasing from one zone to another, is maintained.
 4. Amethod of claim 3, comprising using a ketone as a solvent fortetrafluoroethylene/vinylidene fluoride copolymer.
 5. A method of claim4, comprising using acetone as ketone in the amount of 60-70 weightpercent of said casting solution comprising from 7.0 to 12.0 weightpercent of tetrafluoroethylene/vinylidene fluoride copolymer or mixturethereof with other fluorocarbon polymers.
 6. A method of claim 3,comprisingusing a mixture of alcohol and deionized water as aprecipitating mixture.
 7. A method of claim 3, wherein the alcohol ispropyl alcohol or isopropyl alcohol in the amount of at least 14.0weight percent of said casting solution containing at least 5.0 weightpercent of deionized water.
 8. A method of claim 3, comprisingeffectingdissolution of tetrafluoroethylene/vinylidene fluoride copolymer at thetemperatures ranging from 30° C. to 50° C.
 9. A method of claim 3,comprising,in the course of mixing a copolymer solution with aprecipitating mixture maintaining a difference in temperatures thereofat 3° C. max.
 10. A method of claim 3, comprisingcarrying outimpregnation of porous material with said casting solution by means ofthe application and sizing rollers rotating at various speed and partlysubmerged into said casting solution, and applying said solution ontothe porous support moving at a constant speed within the range of1.0-10.0 m/min.
 11. A method of claim 3, comprising applying a castingsolution onto said porous support in the amount of 10.0-50.0 g ofcopolymer per 1 m² of said support.
 12. A method of claim 3, comprisingmaintaining temperature of a casting solution applied onto said poroussupport within the range of 25° C.-45° C.
 13. A method of claim 3,comprisingperforming short-storage for partial hardening of castingsolution at 18° C.-25° C. for about 0.5-1.0 min.
 14. A method of claim3, comprisingmaintaining temperatures within the drying zones within thefollowing ranges: first zone--ranging from 45° C. to 55° C. secondzone--from 55° C. to 65° C. third zone--from 90° C. to 100° C.andabsolute humidity in each drying zone is maintained at the level of notmore than 5.0 g/m³.
 15. A cartridge filter comprisingprotective andsupporting cylindrical perforated hollow housings, a pleated porousmembrane provided with a prefiltration device and a drainage placedbetween said housings and tightly connected to the cases of end caps, atleast one said end caps having a central drain duct, said cartridgefilter further comprises a membrane consisting of a porous supporthaving an average pore size within the range of 5.0-500.0 microns andthe difference by specific weight in any two points of a sample havingvarious sizes of 7.0% max., which porous support is completelyimpregnated with a copolymer solution soluble in organic solvent whereinthe copolymer solution containing 23-25% by weight tetrafluoroethyleneand 75-77% by weight vinylidene fluoride and having the ratio ofviscosity of the solution thereof in acetone, with copolymer content insolution in the amount of 0.01 g/cm³ to acetone equal to 2.0 to 4.0. 16.A cartridge filter of claim 15, wherein said filter comprises two porousfluorocarbon membranes superimposed so that the one closer to theprefiltration device has the pore size ranging from 0.25 to 0.65 micronsand second membrane has the pore size ranging from 0.15 to 0.45 microns.17. A cartridge filter of claim 16, wherein said filter comprises twoporous membranes superimposed so that active surfaces of said membranesare directed towards the prefiltration device.
 18. A cartridge filter ofclaim 16, wherein said filter comprises two porous membranessuperimposed so that surfaces of said membranes are opposite to activesurfaces.